Norwegian Report Shows Increased Resistance to Sea Lice Drugs

NORWAY - A new report from the Norwegian National Veterinary Institute shows a serious situation in regards to sealice as they are showing more resistance to sealice drugs.

Increasing prescribing and consumption of drugs against sealice has caused decreased susceptibility and more resistance to most drugs (multidrug resistance) along much of the Norwegian coast.

The increase in the number of treatments in 2014 has been greatest in Western Norway.

The report, which was prepared in cooperation with the Norwegian University of Life Sciences on behalf of the Food Safety Authority, shows fully sensitive lice populations are probably only now found in a few areas, in the Southern and Northernmost breeding areas.

Delousing agents are grouped into five categories in the report; pyrethroids, azametifos, emamectin benzoate, hydrogen peroxide and flubenzuroner.

Overall, the prescription data from the veterinary medical register (VetReg) shows an increase in the prescription of drugs. The increase in prescribing treatment lotions are especially great for hydrogen peroxide, flubenzuroner and emamectin benzoate.

Susceptibility testing

Standardized susceptibility tests (simplified bioassay) with delousing agents deltamethrin, azametifos, emamectin benzoate and hydrogen peroxide were conducted on sea lice from a total of 90 farms along the coast.

The results show that reduced sensitivity is widespread and that unlike in 2013, there is now reduced sensitivity to deltamethrin and azametifos also in Finnmark.

Susceptibility testing with hydrogen peroxide shows that lice populations in Trøndelag and Hordaland have reduced sensitivity to hydrogen peroxide, but this is less common along the coast compared with reduced sensitivity to other drugs.

Genotyping (PCR analysis) conducted on lice from farmed areas far south of Norway shows a high degree of sensitivity to pyrethroids and azametifos.

Increased efforts by the FSA

FSA will in 2015 intensify efforts against the irresponsible use of medicines against lice and further facilitate the development and use of non-drug methods.